Lecture: Chapter 23 - Biogeochemical Cycles

Question 1

Biogeochemistry

Answer 1

The study of the physical, chemical and biological factors that influence the movement and transformation of elements.

Question 2

Nutrient Cycles

Answer 2

There are about 100 elements
Approximately 40 are required by organisms
They cycle in the biosphere

Question 3

The most important chemical elements

Answer 3

Carbon
Nitrogen
Phosporus
Sulfur

Question 4

Biogeochemical cycles

Answer 4

Inorganic (nonliving) - Biotic (living)

Elemetns are mostly moved in aqueous solution
Each cycle has a reservoir (atmosphere, ocean, sediments, rock, etc)

Some cycles are more perfect than others. Perfect is in the direction of very rapid transfer of the element to the environment followed by very rapid removal by organisms.

Question 5

How do nutrients enter ecosystems?

Answer 5

Through the chemical breakdown of minerals in rocks or through fixation of atmospheric gases

Question 6

Two basic biogeochemical cycles

Answer 6

Sedimentary / Solid (2.8% of living matter)
26 elements
The solid cycles are less “perfect”

Gaseous (97.2% of living matter)
4 elements : Carbon, hydrogen, oxygen, nitrogen
CHON

Question 7

Solid cycle

Answer 7

All nutrients are ultimately derived from abiotic sources; minerals in rocks and gases in the atmosphere

Question 8

Weathering

Answer 8

Elements are released from rock minerals by weathering physical and chemical breakdown of rocks

Question 9

Atmosphere

Answer 9

The atmosphere si the ultimate source of carbon and nitrogen for ecosytems

• Carbon is taken up as CO2 by autrophs through photosynthesis and fixed into organic compounds
• The atmosphere is 78% nitrogen, in the form of N2, which cannot be used by most organisms because of the energy required to break the triple covalent bond

Question 10

Nitrogen fixation

Answer 10

The process of converting N2 to a biologically useful form

Question 11

Pools

Answer 11

Total amount of a nutrient in a component of the ecosystem

Question 12

Mean residence time (turnover rate)

Answer 12

Amount of time on average that a molecule spends in the pool

Question 13

Where do freshwater and marine ecosytems receive nutrient inputs

Answer 13

Freshwater and marine ecosytems recieve nutrient inputs from terrestrial ecosytems

Question 14

Nutrients in aquatic ecosystems: Lakes

Answer 14

receive nutrients from stream water, atmospheric deposition, fixation and terrestrial litter.

• phosphorus is usually the limiting nutrient in lakes
• Detritus is decomposed int he water column and sediments, providing internal nutrient input
• Nutrients are progressively lost as detritus is deposited in the lake sediments

Question 15

Carbon cycle

Answer 15

Closely linked to energy flow

the largest carbon pool is the ocean

Question 16

Net Ecosystem Production

Answer 16

Production available for accumulation of C/energy witihin the ecosystem

NEP = GPP - (RA+RH+L+D)

Question 17

Rate of Carbon cycle through the ecosytem determined by

Answer 17

• Rate of primary productivity
• Rate of decomposition

Both of these processes are influenced by climate

• temperature
• precipitation

Fastest cycling is in the hot and humid tropics

Question 18

Nitrogen cycle: an atmospheric cycle

Answer 18

The largest nitrogen pool is the atmosphere

The nitrogen cycle is a relatively perfect, self-regulating cycle with a large gaseous reservoir – the atmosphere, which is 78% N

Bacteria are responsible for key steps in the N cycle

The average resident time of fixed N in the biosphere is thought to be about 625 years while for P it is thousands of years.

Question 19

Nitrification

Answer 19

NH3 and NH4+ are converted to NO3- by chemoautotrophic bacteria, in aerobic conditions. It occurs mainly in terrestrial systems. NO3- tends to be more available to plants than other forms of nitrogen

Question 20

Denitrification

Answer 20

Some bacteria use NO3- as an electron acceptor, converting it into N2 and N20, in anoxic conditions

Question 21

Routes of N uptake

Answer 21

Uptake of N2 from atmosphere (very costly energetically)
- N fixtaion: high energy fixation, biological

Uptake of mineral nitrogen from soil

• Ammonium (NH4+) : released by decomposition, created by deposition, affected by cation exchanged capacity
• Nitrate (NO3): deposition, nitrification

Question 22

Factors affecting N release

Answer 22

Decomposition rate influenced by

• water
• temperature
• litter quality
• soil biota

Question 23

Human activity in N release

Answer 23

Human activity greatly increased the amount of N entering the biosphere through agricultural and industrial practices

Question 24

Sources of nitrate: nitrogen deposition

Answer 24

NOx emitted by fossil fuel burning

Some falls as dry deposition

Falls as wet acidic precipitation

Question 25

Nitrogen loss

Answer 25

Leaching

• nitrate is primary form that is lost from ecosystems
• Does not adhere to soils

Fire - volitilization of organic nitrogen

Dentrificiation
- Transformation of nitrate to N2
Requires: lack of oxygen, nitrate

Question 26

Phosophorus cycle

Answer 26

Solid chemical, essential nutrient that moves in sedimentary cycle, which is less perfect than gas cycles.

• vital components of cells in organisms. Biological uses of phosphate: nucleic acids
• P almost always present as phosphate PO4
• Rock is the only source